Recently, a method of applying a carbon-based material as a low-resistance material to an interconnection is extensively studied worldwide. A typical carbon-based material expected to have a low resistance is a carbon nanotube (CNT). The CNT achieves various quantum effects in accordance with the differences between various nanostructures, and can be formed into an insulator, semiconductor, or conductor. Especially when the CNT is formed as a conductor, quantum conduction (ballistic conduction) is expected. This makes the CNT usable as an ultra-low-resistance material replacing the existing metallic material (for example, a Cu interconnection). Also, the CNT can be expected to be advantageous for electrical conduction of a long-distance interconnection because the ballistic length is large.
Unfortunately, it is very difficult to uniformly grow the CNT in the lateral direction (in-plane direction). Therefore, the CNT cannot simply be used as an interconnection material in the in-plane direction. In addition, the CNT is difficult to bend. This makes it impossible to form, for example, an interconnection bending structure by using the CNT, and imposes a large limitation on the layout of patterns.
On the other hand, a method of applying graphene, as a material having a quantum conduction characteristic similar to that of the CNT, to an interconnection material is extensively studied. Graphene is a novel carbon material obtained by extremely thinning graphite. Accordingly, similar to the CNT, graphene is expected to be used as an LSI low-resistance interconnection replacing a metal interconnection due to quantum conduction. Also, graphene has a very large ballistic length, and hence is advantageous for electrical conduction of a long-distance interconnection. Furthermore, since a graphene structure itself is a very thin film (single-layered film), the film can be deposited by chemical vapor deposition (CVD). That is, graphene well matches the formation process of lateral interconnections of devices.
The resistance of a graphene interconnection is determined by the quantum resistance per graphene sheet, and the number of stacked graphene sheets. That is, the resistance increases when the number of stacked graphene sheets is small, and decreases when the number of stacked graphene sheets is large. However, if the number of stacked graphene sheets becomes too large, the interaction between the graphene sheets increases, the mobility of carriers decreases, and the resistance increases.
In addition, since the bond between graphene sheets is different from that in the plane of a graphene sheet, electrical conduction between graphene sheets may be different from that in the plane of a graphene sheet. That is, electrical conduction between graphene sheets may have a resistance higher than that of electrical conduction in the plane of a graphene sheet.
As described above, demands have arisen for a further decrease in resistance of the graphene interconnection.